1. Field of Invention
This invention relates to in the field of amplification of high frequency communications signals.
2. Description of Related Art
Low Noise Amplifiers (LNAs) are well known in the art of amplifying weak signals in wireless communications systems. For example, LNAs were used in telephone systems including time division multiple access (TDMA) systems and code division multiple access (CDMA) systems. Since these systems were primarily directed to low data rate voice transmissions relatively narrow bandwidth amplifiers were adequate. For example, the frequency bandwidths that were amplified in these systems typically extended only to approximately 5 MHz.
However, wide band LNAs are becoming increasingly important for a number of reasons. One reason is the fact that a new ultra-wide band (UWB) standard covering the range of approximately 3 GHz to 10 GHz has come into effect. See. The new standard was mainly directed to high data rate wireless transmissions. Furthermore, wide band LNAs could be applied to many of the popular new hand held and non hand held devices. Another reason LNAs became increasingly important was the fact they could be applied to the antennas of many devices for receiving and processing weak signals. For example, multimode receivers and software defined radio could require wide band LNAs in order to amplify the weak signals from their antennas.
Therefore, in order to comply with the new standards UWB LNAs were required to meet much more demanding specifications. Complying with the new standards required designers to extend the bandwidth of LNAs in the areas of input impedance matching, noise figure, gain and linearity. Chip areas were required to be small in order to reduce the cost of manufacturing the LNAs. Additionally, since UWB LNAs could be used in battery powered applications such as hand held devices it was preferable that they have low power requirements.
It was known to use both input matching networks and output load bandwidth extension techniques in attempting to meet these requirements. For example, it was known to use multistage input matching networks to attempt to obtain the input matching suitable for the bandwidths required in the new applications. In this manner, a bandpass response of several octaves could be achieved. However, several inductors were required to implement the multistage matching networks filters that were suitable for the new standards. Furthermore, in cases where higher order matching networks were required, additional inductors were needed. Since inductors can occupy a large chip area, multistage filters using the foregoing inductors were often too large and expensive to manufacture economically.
It was also known to use distributed amplifiers in an attempt to obtain the wide required bandwidth. However, distributed amplifiers required the cascading of multiple stages of amplifiers and the use many inductors. Furthermore, multiple stages of amplifiers required large power consumption and inductors required a lot of chip area. These two drawbacks made the distributed amplifier unsuitable for designing wide band LNAs.
Additionally, it was known to add an inductor to the output load of an LNA in order to extend the gain bandwidth of LNAs. However, the load inductors used in this manner were undesirably large, as previously described. In another attempt to meet the new standards, resistive feedback was provided at the drain and gate nodes of the input transistors of the LNAs to improve input impedance matching and extend bandwidth. However, none of the known techniques for designing UWB LNAs could meet all of the specifications required for the new standards economically.